A typical vane-type camshaft phaser for changing the phase relationship between a crankshaft and a camshaft of an internal combustion engine generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is selectively supplied to one of the advance and retard chambers and vacated from the other of the advance and retard chambers by a phasing oil control valve in order to rotate the rotor within the stator and thereby change the phase relationship between the camshaft and the crankshaft. One such camshaft phaser is described in U.S. Pat. No. 8,534,246 to Lichti et al., the disclosure of which is incorporated herein by reference in its entirety and hereinafter referred to as Lichti et al.
While the camshaft phaser of Lichti et al. may be effective, the camshaft phaser may be parasitic on the lubrication system of the internal combustion engine which also supplies the oil for rotating the rotor relative to the stator, thereby requiring increased capacity of an oil pump of the internal combustion engine which adds load to the internal combustion engine. In an effort to reduce the parasitic nature of camshaft phasers, so-called cam torque actuated camshaft phasers have also been developed. In a cam torque actuated camshaft phaser, oil is moved directly from the advance chambers to the retard chambers or directly from the retard chambers to the advance chambers based on torque reversals imparted on the camshaft from intake and exhaust valves of the internal combustion engine. The torque reversals are predictable and cyclical in nature and alternate from tending to urge the rotor in the advance direction to tending to urge the rotor in the retard direction. The effects of the torque reversals on oil flow are known to be controlled by a valve spool positioned by a solenoid actuator. Accordingly, in order to advance the camshaft phaser, the valve spool is positioned by the solenoid actuator to create a passage with one or more check valves which allow torque reversals to transfer oil from the advance chambers to the retard chambers while preventing torque reversals from transferring oil from the retard chambers to the advance chambers. Conversely, in order to retard the camshaft phaser, the valve spool is positioned by the solenoid actuator to create a passage with the one or more check valves which allow torque reversals to transfer oil from the retard chambers to the advance chambers while preventing torque reversals from transferring oil from the advance chambers to the retard chambers. One such camshaft phaser is described in U.S. Pat. No. 7,000,580 to Smith et al., hereinafter referred to as Smith et al. Smith et al. teaches an arrangement which uses two check valves located within a valve spool in order to allow oil to flow from the chambers which need to decrease in volume to the chambers which need to increase in volume while preventing oil flow in the reverse direction. In operation, when torque reversals of the camshaft cause oil to tend flow in the reverse direction, high pressure oil is applied only to one check valve. Consequently, high pressure oil from the reversing torque reversal is applied to a large volume which requires substantial structure to resist the high oil pressure.
Another such cam torque actuated camshaft phaser is described in U.S. Pat. No. 7,137,371 to Simpson et al., hereinafter referred to as Simpson et al. Simpson et al. differs from Smith et al. in that Simpson et al. requires only one check valve to transfer oil from the advance chambers to the retard chambers and to transfer oil from the retard chambers to the advance chambers. While Simpson et al. eliminates one check valve compared to Smith et al., the passages of Simpson et al. that are required to implement the single check valve add further complexity because the check valve is located remotely from the valve spool.
Yet another such cam torque actuated camshaft phaser is described in United States Patent Application Publication No. US 2013/0206088 A1 to Wigsten, hereinafter referred to as Wigsten. Wigsten differs from Simpson et al. in that the check valve that is used to transfer oil from the advance chambers to the retard chambers and to transfer oil from the retard chambers to the advance chambers is located within the valve spool. However, placement of the check valve within the valve spool as implemented by Wigsten complicates the manufacture of the valve spool and adds further complexity to passages needed in the valve body within which the valve spool is slidably disposed.
What is needed is camshaft phaser which minimizes or eliminates one or more the shortcomings as set forth above.